Aqueous dispersions containing vinyl ester polymers are well known and are especially useful in the adhesive industry. For such uses, it is common to polymerize the monomer in an aqueous medium containing polymerization initiators and water soluble colloids. The most commonly used colloid is polyvinyl alcohol which has been hydrolyzed to a level of about 88%, i.e., which contains about 12% vinyl acetate. The resulting emulsion polymer is then formulated with either partially or fully hydrolyzed polyvinyl alcohol to obtain adhesive compositions having varying properties.
In recent years, there has emerged an increasing need for higher solids emulsions which will provide faster setting times for use on high speed production equipment. These high solids adhesive bases also find use as replacements for conventional hot melt (100% solids) material which require elevated temperatures with consequent expenditures of considerable energy. In addition to the need for high solids emulsions for such applications, it is also essential that the emulsion remain sufficiently low in viscosity that they can be applied using conventional equipment. Typically desirable coating viscosities are in the range of 800-5000 cps at 25.degree. C.
Various methods have been described for the preparation of vinyl acetate based emulsions at solids levels of about 60%-65%. These methods are described for example in U.S. Pat. Nos. 4,921,898, 5,070,134, 5,629,370 and EP389893.
It is an object of the present invention to provide a process for the production of vinyl acetate based homo and copolymer emulsions at high solids levels, i.e., in excess of 65% and preferably greater than 70% solids while maintaining relatively low coating viscosities in the range of 800 to 5000 cps at 25.degree. C.